Poly(hydroxyalkanoic acids), also known as polyhydroxyalkanoates, have gained great attention in the plastics industry because they can be produced from renewable monomers. Because these polymers are biodegradable they have been proposed for use in a broad range of applications. However, physical limitations such as brittleness and slow crystallization may limit the applications of poly(hydroxyalkanoic acids). Numerous impact modifiers have been developed in the past to improve the toughness of poly(hydroxyalkanoic acids). For example, Japanese Patent No. 9316310 discloses a poly(lactic acid) resin composition comprising a modified olefin compound as an impact modifier. U.S. Patent Application Publication No. 2006/0173133 discloses a toughened poly(hydroxyalkanoic acid) composition wherein an ethylene ester copolymer (e.g., a terpolymer having copolymerized units of ethylene, butyl acrylate and glycidyl methacrylate (EBAGMA)) is used as an impact modifier. In addition, various nucleating agents have been developed to increase the crystallinity or rate of crystallization for poly(hydroxyalkanoic acid) compositions and therefore improve the dimensional stability and thermal resistance thereof. For example, U.S. Pat. No. 6,417,294 discloses the use of an aliphatic carboxylic acid amide (e.g., behenamide) as a nucleating agent for poly(lactic acid).
In general, when poly(hydroxyalkanoic acid) is used to manufacture thermoformed packaging articles, such as cups, trays, and clam shells, the resin is first melt extruded and rapidly cooled (i.e. generally for less than 5 seconds) to a temperature below the glass transition temperature of the polymer on a rotating quench drum to form a nearly transparent sheet (herein called Rapidly Quenched Sheet). The sheet is then further formed into finished articles under higher temperature conditions (above the glass transition temperature of the polymer). The term “Rapidly Quenched Sheet” as used herein also encompasses thin sheets, i.e. films, for example of thickness of 10 mils (0.25 mm) or less.
Rapidly Quenched Sheet of poly(hydroxyalkanoic acid), however, often lacks the stretchability that is required for acceptable processability on continuously moving equipment. Thus, there is a need for a poly(hydroxyalkanoic acid) containing composition that not only has desirable toughness and dimensional stability, but that also has adequate stretchability, especially when in the form of a Rapidly Quenched Sheet.